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Drug Reference

omeprazole

Proton pump inhibitors· Brands: Losec, Prilosec, Omez, AntraIVPO

Mechanism of action
Omeprazole is a proton pump inhibitor that irreversibly inhibits the gastric H+/K+ ATPase enzyme (the proton pump) in the parietal cells of the stomach, thereby blocking the final step of gastric acid secretion. This results in a profound and long-lasting reduction of both basal and stimulated acid production.
Pharmacokinetics
Omeprazole is a weak base that is acid-labile and therefore administered as an enteric-coated formulation. It is well absorbed from the small intestine, with peak plasma concentrations occurring 0.5 to 3.5 hours after oral administration. It undergoes extensive first-pass metabolism via hepatic cytochrome P450 (CYP2C19 and CYP3A4). Plasma half-life is short (0.5-1 hour), but the antisecretory effect persists for 24-48 hours due to irreversible binding of the proton pump. It is metabolised primarily in the liver and excreted as inactive metabolites in urine and faeces.
Indications & dosing

Gastro-oesophageal reflux disease (GORD)

For severe oesophagitis, use 40 mg once daily for 4 weeks.

  • adult · PO: 20–40 mg once daily · For severe oesophagitis, use 40 mg once daily for 4 weeks.

Peptic ulcer disease (gastric or duodenal)

Usually for 4 weeks, or 8 weeks for gastric ulcer.

  • adult · PO: 20 mg once daily · Usually for 4 weeks, or 8 weeks for gastric ulcer.

Helicobacter pylori eradication (in combination therapy)

Given with antibiotics, e.g., amoxicillin and clarithromycin for 7-14 days.

  • adult · PO: 20 mg twice daily · Given with antibiotics, e.g., amoxicillin and clarithromycin for 7-14 days.

Prevention of NSAID-induced ulcers

For patients at high risk requiring ongoing NSAID therapy.

  • adult · PO: 20 mg once daily · For patients at high risk requiring ongoing NSAID therapy.

Zollinger-Ellison syndrome

Dose titrated to suppress gastric acid secretion; doses above 80 mg/day should be divided.

  • adult · PO: 60 mg once daily · Dose titrated to suppress gastric acid secretion; doses above 80 mg/day should be divided.

Dyspepsia

Short-term use; review if symptoms persist after 2 weeks.

  • adult · PO: 10–20 mg once daily · Short-term use; review if symptoms persist after 2 weeks.
Contraindications & precautions
  • Hypersensitivity to omeprazole, other proton pump inhibitors, or any excipient · Cross-sensitivity may occur among PPIs.
  • Concomitant use with nelfinavir · Omeprazole significantly decreases nelfinavir plasma concentrations, reducing its antiviral efficacy.
  • Severe hepatic impairment · Omeprazole is extensively metabolised in the liver; caution and dose adjustment may be required.
  • Concomitant use with clopidogrel · Omeprazole inhibits CYP2C19, reducing clopidogrel's active metabolite and antiplatelet effect.
  • Use the lowest effective dose for the shortest duration; review the need for continued therapy regularly.
  • Evaluate for alarm symptoms (weight loss, dysphagia, vomiting, anaemia) to exclude gastric malignancy before initiating treatment.
  • Monitor for signs of C. difficile-associated diarrhoea, particularly in hospitalised or immunocompromised patients.
  • Use with caution in patients with hepatic impairment; consider a reduced dose.
  • Be aware of potential interaction with clopidogrel; consider alternative acid suppression (e.g. pantoprazole) if antiplatelet therapy is needed.
  • In long-term use, consider monitoring serum magnesium and vitamin B12 levels, and discuss bone health.
Adverse effects
  • Headache
  • Abdominal pain
  • Diarrhoea
  • Nausea
  • Flatulence
  • Clostridioides difficile-associated diarrhoeaserious
  • Interstitial nephritisserious
  • Hypomagnesemiaserious
  • Vitamin B12 deficiencyserious
  • Fracture risk (hip, wrist, spine)serious
Interactions
  • dabigatranmoderateUse with caution. Monitor patients for signs and symptoms of bleeding. Consider an alternative proton pump inhibitor with less P-gp inhibition (e.g., pantoprazole) if feasible. If coadministration is necessary, no routine dose adjustment is recommended, but periodic clinical assessment and renal function monitoring are advised.
  • warfarinmoderateMonitor INR closely, especially when starting or stopping omeprazole. Consider alternative acid-suppressant therapy with a proton pump inhibitor that has less CYP2C9/2C19 interaction (e.g., pantoprazole) if appropriate. Adjust warfarin dose as required.
  • clopidogrelmajorAvoid the combination. If gastroprotection is required, consider an H2-receptor antagonist such as famotidine or a proton pump inhibitor with less CYP2C19 inhibition, such as pantoprazole. If omeprazole is considered necessary, weigh the risk of reduced clopidogrel efficacy and monitor for signs of thrombotic events, but no reliable laboratory measure exists to adjust therapy.
  • methotrexatemajorAvoid concurrent use of omeprazole with high-dose methotrexate. If co-administration is unavoidable, monitor methotrexate plasma levels, renal function, and full blood counts closely. Consider alternative acid-suppressive therapy such as an H2 receptor antagonist if required.
  • levothyroxinemoderateMonitor serum TSH and free T4 levels after initiating or discontinuing omeprazole, especially in patients on long-term levothyroxine. Advise patients to take levothyroxine at least 4 hours apart from omeprazole if possible, although gastric pH effects may persist. Adjust levothyroxine dose based on thyroid function tests. Consider alternative acid-suppressants if clinically appropriate, or increase monitoring frequency.
  • phenytoinmoderateMonitor plasma phenytoin concentrations and observe for signs of toxicity when omeprazole is started, stopped, or the dose is changed. Consider reducing the phenytoin dose if levels rise. If clinically appropriate, an alternative proton pump inhibitor with less CYP2C19 inhibition, such as pantoprazole, could be substituted.
Renal & hepatic adjustment
  • Hepatic impairment (mild): No dose adjustment required.
  • Hepatic impairment (moderate): No dose adjustment required.
  • Hepatic impairment (severe): Reduce dose to a maximum of 20 mg once daily.
Monitoring, pregnancy & toxicity
  • Symptom resolution · Assess after 2 weeks of therapy or at the end of treatment
  • H. pylori eradication · If used as part of eradication regimen, confirm at least 4 weeks after treatment
  • Serum magnesium · Baseline and periodically during long-term use
  • Serum vitamin B12 · Consider during long-term therapy
  • Pregnancy: May be used when clinically indicated. Data suggest no increased risk of congenital malformations.
  • Toxicity: Overdose is usually benign, with mild gastrointestinal symptoms such as nausea, vomiting, abdominal pain, and diarrhoea. · Supportive care. No specific antidote. Consider activated charcoal if a large recent ingestion.
Clinical notes
Omeprazole is used for gastro-oesophageal reflux disease (GORD), peptic ulcer disease, Helicobacter pylori eradication (in combination with antibiotics), and Zollinger-Ellison syndrome. It is also used for prophylaxis of NSAID-induced ulcers. Long-term use may be associated with vitamin B12 deficiency, hypomagnesemia, increased risk of fractures, and Clostridium difficile infection. It may interact with clopidogrel, reducing its activation. Doses should be adjusted in hepatic impairment. Ideally taken before the first meal of the day.

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BNF; local formulary